Pace pulse detector for a medical device

ABSTRACT

In situations in which an implantable medical device (e.g., a subcutaneous ICD) is co-implanted with a leadless pacing device (LPD), it may be important that the subcutaneous ICD knows when the LPD is delivering pacing, such as anti-tachycardia pacing (ATP). Techniques are described herein for detecting, with the ICD and based on the sensed electrical signal, pacing pulses and adjusting operation to account for the detected pulses, e.g., blanking the sensed electrical signal or modifying a tachyarrhythmia detection algorithm. In one example, the ICD includes a first pace pulse detector configured to obtain a sensed electrical signal and analyze the sensed electrical signal to detect a first type of pulses having a first set of characteristics and a second pace pulse detector configured to obtain the sensed electrical signal and analyze the sensed electrical signal to detect a second type of pulses having a second set of characteristics.

This application is a Continuation of U.S. patent application Ser. No.14/687,053, filed Apr. 15, 2015, entitled “PACE PULSE DETECTOR FOR ANIMPLANTABLE MEDICAL DEVICE,” which claims the benefit of the filing dateof provisional U.S. Patent Application No. 61/984,249, filed on Apr. 25,2014, entitled “PACE PULSE DETECTOR FOR AN IMPLANTABLE MEDICAL DEVICE,”the content of both of which is incorporated herein by reference intheir entirety.

TECHNICAL FIELD

This application relates to medical devices configured to detect andtreat cardiac arrhythmias.

BACKGROUND

ICD systems may be used to deliver high energy cardioversion ordefibrillation shocks to a patient's heart to terminate a detectedtachyarrhythmia, such as an atrial or ventricular fibrillation.Cardioversion shocks are typically delivered in synchrony with adetected R-wave when fibrillation detection criteria are met.Defibrillation shocks are typically delivered when fibrillation criteriaare met, and the R-wave cannot be discerned from signals sensed by theICD. Additionally, ICD systems may also deliver high energycardioversion or defibrillation shocks to terminate certain types ofventricular tachycardia (VT).

ICD systems generally include an ICD that is coupled to one or moreelectrical leads placed within or attached to the heart. The electricalleads include one or more electrodes positioned in or on the heart bythe leads and used for therapy and/or sensing functions. Cardioversionand defibrillation shocks (e.g., anti-tachyarrhythmia or high voltageshocks) are generally applied between a coil electrode carried by one ofthe leads and the ICD housing, which acts as an active can electrode.

In addition, or as an alternative to cardioversion and defibrillationshocks, the ICD system may provide pacing therapy to the heart.Conventional ICD systems provide the pacing therapy via the electrodesof the lead that are positioned near or against the cardiac tissue toprovide sufficient transmission of electrical energy to the cardiactissue in order to capture the heart. The pacing therapy may, forexample, include cardiac pacing to suppress or convert tachyarrhythmiasto sinus rhythm. Such pacing is often referred to as anti-tachycardiapacing or ATP. The ICD system may provide ATP in an attempt to terminatearrhythmias that would otherwise need to be treated by a cardioversionor defibrillation shock, which are uncomfortable for the patient. TheICD system may also provide anti-bradycardia pacing when the naturalpacemaker and/or conduction system of the heart fails to providesynchronized atrial and ventricular contractions at rates and intervalssufficient to sustain healthy patient function.

SUMMARY

Subcutaneous ICD systems have also been developed that do not includeleads that are within or attached to the heart. In a subcutaneous ICDsystem, the lead is instead placed subcutaneously above the ribcageand/or sternum. Such systems do not generally provide ATP because of theamount of energy required for such pacing pulses as well as thediscomfort experienced by the subject in which the device is implanted.Systems have been proposed in which a leadless pacing device (LPD) orother artificial pacemaker is implanted along with the subcutaneous ICDto provide the desired ATP.

In situations in which a subcutaneous ICD operates in conjunction with aco-implanted LPD it may be important that the subcutaneous ICD knowswhen pacing, such as ATP, is being or has been delivered by the LPD.Based on the knowledge that pacing is being or has been delivered, thesubcutaneous ICD may make some sort of adjustment to account for thepacing. For example, the subcutaneous ICD may blank the sensing channelto remove the pacing pulse from the sensed electrical signal, adjust atachyarrhythmia detection algorithm, make another adjustment, or acombination thereof.

In one example, this disclosure is directed to an implantable medicaldevice comprising a first pace pulse detector configured to obtain asensed electrical signal and analyze the sensed electrical signal todetect a first type of pulses having a first set of characteristics anda second pace pulse detector configured to obtain the sensed electricalsignal and analyze the sensed electrical signal to detect a second typeof pulses having a second set of characteristics.

In another example, this disclosure provides a method comprisingprocessing electrical signals sensed on an implantable electrical leadcoupled to an implantable medical device, detecting, based on theprocessing of the electrical signals sensed on the implantableelectrical lead, delivery of pacing pulses from a second implantablemedical device, wherein the detecting comprises detecting a first typeof pulse having a first set of characteristics and a second type ofpulse having a second set of characteristics different than the firstset, modifying the sensed electrical signal to remove the first type ofpulse from the sensed electrical signal, and modifying a tachyarrhythmiadetection algorithm based on the detected pacing pulses.

In a further example, this disclosure is directed to an extravascularimplantable cardioverter-defibrillator (ICD) comprising a sensing modulethat obtains electrical signals sensed using at least an implantableelectrical lead coupled to the ICD, a pace detector including a firstpace pulse detector that detects, within the obtained electricalsignals, a first type of pulses having a first set of characteristicsand a second pace pulse detector that detects, within the obtainedelectrical signals, a second type of pulses having a second set ofcharacteristics different than the first set of characteristics, ablanking module that holds the sensed electrical signal at a currentvalue to remove the first type of pulse from the sensed electricalsignal, and a control module that modifies a tachyarrhythmia detectionalgorithm based on the detected pacing pulses.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual drawing illustrating an example cardiac systemhaving coexistent ICD system and pacing system implanted within apatient.

FIG. 2 is a functional block diagram of an example configuration ofelectronic components of an example ICD.

FIG. 3 is a block diagram of an example sensing channel of a sensingmodule of an ICD with pace detection and removal.

FIG. 4A illustrates a plot of an ECG of a ventricular tachycardia withpacing spikes.

FIG. 4B illustrates a plot representing operations performed on the ECGand occurring within sense digital filter showing the impact of pacingartifacts on sensing performance.

FIG. 5A illustrates a plot of the ECG of FIG. 4A after being modified toremove pacing pulses.

FIG. 5B illustrates a plot representing operations performed by sensedigital filter on the ECG after modifying a sensed electrical signal toremove pacing pulses.

FIG. 6 is block diagram illustrating an example pulse detector.

FIG. 7 is a conceptual diagram of a pace pulse detector analyzing theslew rate of the sensed electrical signals having pacing pulses.

FIG. 8 is block diagram illustrating another example pulse detector.

FIG. 9 is a flow diagram illustrating example operation of a sensingchannel controlling the modification of the sensed electrical signal toremove pacing pulses from signals on one or more sensing channels basedon input indicative of a pacing pulse.

FIG. 10 is a state diagram of an example tachyarrhythmia detectionalgorithm.

FIG. 11 is a flow diagram illustrating example operation of a controlmodule detecting a pacing train and modifying tachyarrhythmia detectionin response to detecting the pacing train.

FIG. 12 is a flow diagram illustrating example operation of a controlmodule implementing a modified tachyarrhythmia detection algorithm toaccount for ATP.

FIG. 13 is a flow diagram illustrating example operation of a controlmodule modifying a tachyarrhythmia detection algorithm to account forfast bradycardia pacing.

DETAILED DESCRIPTION

FIG. 1 is a conceptual drawing illustrating an example cardiac system 10implanted within a patient 12. Cardiac system 10 includes a subcutaneousICD system 14 implanted above the ribcage and sternum and a leadlesscardiac pacing device 16 implanted within a heart 18 of patient 12. Aswill be described in further detail herein, subcutaneous ICD system 14is configured to detect pacing therapy delivered by pacing device 16 byanalyzing sensed electrical signals and, in response to detecting thepacing therapy, modify sensing and/or tachyarrhythmia detection.

Subcutaneous ICD system 14 includes an implantable cardiac defibrillator(ICD) 20 connected to at least one implantable cardiac defibrillationlead 22. ICD 20 of FIG. 1 is implanted subcutaneously on the left sideof patient 12 under the skin but above the ribcage. Defibrillation lead22 extends subcutaneously under the skin but above the ribcage from ICD20 toward a center of the torso of patient 12, bends or turns near thecenter of the torso, and extends subcutaneously superior under the skinbut above the ribcage and/or sternum. Defibrillation lead 22 may beoffset laterally to the left or the right of the sternum or located overthe sternum. Defibrillation lead 22 may extend substantially parallel tothe sternum or be angled lateral from the sternum at either the proximalor distal end.

Defibrillation lead 22 includes an insulative lead body having aproximal end that includes a connector configured to be connected to ICD20 and a distal portion that includes one or more electrodes.Defibrillation lead 22 also includes one or more conductors that form anelectrically conductive path within the lead body and interconnect theelectrical connector and respective ones of the electrodes.

Defibrillation lead 22 includes a defibrillation electrode 24 toward thedistal portion of defibrillation lead 22, e.g., toward the portion ofdefibrillation lead 22 extending along the sternum. Defibrillation lead22 is placed along sternum such that a therapy vector betweendefibrillation electrode 24 and a housing electrode formed by or on ICD20 (or other second electrode of the therapy vector) is substantiallyacross a ventricle of heart 18. The therapy vector may, in one example,be viewed as a line that extends from a point on defibrillationelectrode 24 (e.g., a center of the defibrillation electrode 24) to apoint on the housing electrode of ICD 20. Defibrillation electrode 24may, in one example, be an elongated coil electrode.

Defibrillation lead 22 may also include one or more sensing electrodes,such as sensing electrodes 26 and 28, located along the distal portionof defibrillation lead 22. In the example illustrated in FIG. 1, sensingelectrodes 26 and 28 are separated from one another by defibrillationelectrode 24. In other examples, however, sensing electrodes 26 and 28may be both distal of defibrillation electrode 24 or both proximal ofdefibrillation electrode 24. In other examples, lead 22 may include moreor fewer electrodes.

ICD system 14 may sense electrical signals via one or more sensingvectors that include combinations of electrodes 26 and 28 and thehousing electrode of ICD 20. For example, ICD 20 may obtain electricalsignals sensed using a sensing vector between electrodes 26 and 28,obtain electrical signals sensed using a sensing vector betweenelectrode 26 and the conductive housing electrode of ICD 20, obtainelectrical signals sensed using a sensing vector between electrode 28and the conductive housing electrode of ICD 20, or a combinationthereof. In some instances, ICD 20 may even sense cardiac electricalsignals using a sensing vector that includes defibrillation electrode 24and one of electrodes 26 and 28 or the housing electrode of ICD 20.

The sensed electrical intrinsic signals may include electrical signalsgenerated by cardiac muscle and indicative of depolarizations andrepolarizations of heart 18 at various times during the cardiac cycle.Additionally, the sensed electrical signals may also include electricalsignals, e.g., pacing pulses, generated and delivered to heart 18 bypacing device 16. ICD 20 analyzes the electrical signals sensed by theone or more sensing vectors to detect tachyarrhythmia, such asventricular tachycardia or ventricular fibrillation. In response todetecting the tachycardia, ICD 20 may begin to charge a storage element,such as a bank of one or more capacitors, and, when charged, deliver oneor more defibrillation shocks via defibrillation electrode 24 ofdefibrillation lead 22 if the tachyarrhythmia is still present anddetermined to require defibrillation therapy. As will be described infurther detail herein, ICD 20 analyzes the sensed electrical signals onlead 22 to detect pacing therapy provided by pacing device 16 and, inresponse to detecting the pacing therapy, modifies the sensing and/ortachyarrhythmia detection to reduce the likelihood that the pacingtherapy negatively impacts the sensing and detection of ICD 20.

As described above, cardiac system 10 also includes at least one cardiacpacing device 16. In the example illustrated in FIG. 1, cardiac pacingdevice 16 is an implantable leadless pacing device that provides pacingtherapy to heart 18 via a pair of electrodes carried on the housing ofpacing device 16. An example cardiac pacing device is described in U.S.patent application Ser. No. 13/756,085 to Greenhut et al., entitled“SYSTEMS AND METHODS FOR LEADLESS PACING AND SHOCK THERAPY,” the entirecontent of which is incorporated herein by reference. Since cardiacpacing device 16 includes two or more electrodes carried on the exteriorits housing, no other leads or structures need to reside in otherchambers of heart 18.

In the example of FIG. 1, cardiac pacing device 16 is implanted withinright ventricle of heart 18 to sense electrical activity of heart 18 anddeliver pacing therapy, e.g., anti-tachycardia pacing (ATP) therapy,bradycardia pacing therapy, and/or post-shock pacing therapy, to heart18. Pacing device 16 may be attached to a wall of the right ventricle ofheart 18 via one or more fixation elements that penetrate the tissue.These fixation elements may secure pacing device 16 to the cardiactissue and retain an electrode (e.g., a cathode or an anode) in contactwith the cardiac tissue. However, in other examples, system 10 mayinclude additional pacing devices 16 within respective chambers of heart12 (e.g., right or left atrium and/or left ventricle). In furtherexamples, pacing device 16 may be attached to an external surface ofheart 18 (e.g., in contact with the epicardium) such that pacing device16 is disposed outside of heart 18.

Pacing device 16 may be capable sensing electrical signals using theelectrodes carried on the housing of pacing device 16. These electricalsignals may be electrical signals generated by cardiac muscle andindicative of depolarizations and repolarizations of heart 18 at varioustimes during the cardiac cycle. Pacing device 16 may analyze the sensedelectrical signals to detect tachyarrythmias, such as ventriculartachycardia or ventricular fibrillation. In response to detecting thetachyarrhythmia, pacing device 16 may, e.g., depending on the type oftachyarrhythmia, begin to deliver ATP therapy via the electrodes ofpacing device 16. In addition to or instead of ATP therapy, pacingdevice 16 may also deliver bradycardia pacing therapy and post-shockpacing therapy.

Cardiac pacing device 16 and subcutaneous ICD system 14 are configuredto operate completely independent of one another. In other words, pacingdevice 16 and subcutaneous ICD system 14 are not capable of establishingtelemetry communication sessions with one another to exchangeinformation about sensing and/or therapy using one-way or two-waycommunication. Instead, each of pacing device 16 and subcutaneous ICDsystem 14 analyze the data sensed via their respective electrodes tomake tachyarrhythmia detection and/or therapy decisions. As such, eachdevice does not know if the other will detect the tachyarrhythmia, if orwhen it will provide therapy, and the like.

During a tachyarrhythmia that could be treated with either ATP or adefibrillation shock, it is important to ensure that ATP therapies donot overlap or take place after the defibrillation shock. Applying ATPafter a defibrillation shock could be pro-arrhythmic and present ahazard to the patient. Moreover, the delivery of the pacing from pacingdevice 16 could interference with sensing and tachyarrhythmia detectionof subcutaneous ICD 20. This interference could take the form ofdecreased sensitivity (e.g., inability to detect ventricular tachycardia(VT) and/or ventricular fibrillation (VF)) or decreased specificity(e.g., inability to withhold therapy for tachyarrhythmia's determined tonot require a defibrillation shock, such as supraventricular tachycardia(SVT), sinus tachycardia (ST), normal sinus rhythm, atrial fibrillation,atrial flutter, or the like). Systems could be designed to providedevice-to-device communication between subcutaneous ICD system 14 andpacing device 16, but this may add complexity to the system and not behighly effective or fast enough to prevent unwanted ATP therapies postdefibrillation shock. The techniques described herein reduce and, insome cases, eliminate the interference with sensing and tachyarrhythmiadetection of subcutaneous ICD 20.

Although FIG. 1 is described in the context of a subcutaneous ICD system14 and a leadless pacing device 16, the techniques may be applicable toother coexistent systems. For example, an ICD system that includes alead having a distal portion that is implanted at least partially underthe sternum (or other extra-pericardial location) instead of beingimplanted above the ribs and/or sternum. As another example, instead ofa leadless pacing device, a pacing system may be implanted having apacemaker and one or more leads connected to and extending from thepacemaker into one or more chambers of the heart or attached to theoutside of the heart to provide pacing therapy to the one or morechambers. Moreover, the techniques of this disclosure may additionallybe useful in implantable medical systems that do not include an ICD 20.For example, it may be beneficial for leadless pacing devices implantedin different chambers of the heart to be able to detect pacing pulsesdelivered by each other so that no direct communication is necessary. Assuch, the example of FIG. 1 is illustrated for exemplary purposes onlyand should not be considered limiting of the techniques describedherein.

FIG. 2 is a functional block diagram of an example configuration ofelectronic components of an example ICD 20. ICD 20 includes a controlmodule 30, sensing module 32, therapy module 34, communication module38, and memory 40. The electronic components may receive power from apower source 36, which may, for example, be a rechargeable ornon-rechargeable battery. In other embodiments, ICD 20 may include moreor fewer electronic components. The described modules may be implementedtogether on a common hardware component or separately as discrete butinteroperable hardware, firmware or software components. Depiction ofdifferent features as modules is intended to highlight differentfunctional aspects and does not necessarily imply that such modules mustbe realized by separate hardware, firmware or software components.Rather, functionality associated with one or more modules may beperformed by separate hardware, firmware or software components, orintegrated within common or separate hardware, firmware or softwarecomponents.

Sensing module 32 is electrically coupled to some or all of electrodes24, 26, and 28 via conductors of lead 22 and one or more electricalfeedthroughs, and is also electrically coupled to the housing electrodevia conductors internal to the housing of ICD 20. Sensing module 32 isconfigured to obtain electrical signals sensed via one or morecombinations of electrodes 24, 26, and 28, and the housing electrode ofICD 20, and process the obtained electrical signals.

Sensing module 32 may include one or more analog components, digitalcomponents or a combination thereof. Sensing module 32 may convert thesensed signals to digital form and provide the digital signals tocontrol module 30 for processing or analysis. For example, sensingmodule 32 may amplify signals from the sensing electrodes and convertthe amplified signals to multi-bit digital signals using ananalog-to-digital converter (ADC). Sensing module 32 may also compareprocessed signals to a threshold to detect the existence of atrial orventricular depolarizations (e.g., P- or R-waves) and indicate theexistence of the atrial depolarization (e.g., P-waves) or ventriculardepolarizations (e.g., R-waves) to control module 30. Sensing module 32may also process the sensed signals to output an electrocardiogram tocontrol module 30.

Control module 30 may process the signals from sensing module 32 tomonitor for a tachyarrhythmia, such as VT or VF. In response todetecting the tachyarrhythmia, control module 30 may control therapymodule 34 to charge a storage element within therapy module 34, and,when necessary, deliver a cardioversion or defibrillation pulse toterminate the tachyarrhythmia. The cardioversion or defibrillation pulsemay be provided using a therapy vector between defibrillation electrode24 of lead 22 and the housing electrode of ICD 20. Therapy module 34may, for example, include one or more capacitors, transformers,switches, and the like. Control module 30 may control therapy module 34to generate and deliver cardioversion or defibrillation shocks havingany of a number of waveform properties, including leading-edge voltage,tilt, delivered energy, pulse phases, and the like.

As described above with respect to FIG. 1, pacing device 16independently detects a tachyarrhythmia and, in some instances, providesATP in an attempt to terminate the tachyarrhythmia. The ATP therapyprovided by pacing device 16 may interfere with sensing and detection oftachyarrhythmia by sensing module 32 of ICD 20. This interference couldtake the form of decreased sensitivity (e.g., inability to detect VT orVF) or decreased specificity (e.g., detecting VT or VF for rhythms inwhich no therapy is necessary). ICD 20 is configured to detect the ATPprovided by pacing device 16 by analyzing the sensed electrical signalsfrom lead 22 and, adjust sensing and/or detection in response todetecting the ATP. To this end, sensing module 32 may include additionalcomponents configured to detect pacing spikes within the sensedelectrical signals from lead 22. For example, sensing module 32 mayinclude a pace pulse detector as described in further detail withrespect to FIGS. 3 and 5.

Communication module 38 includes any suitable hardware, firmware,software or any combination thereof for communicating with an externaldevice, such as a clinician programmer or patient monitoring device. Forexample, communication module 38 may include appropriate modulation,demodulation, frequency conversion, filtering, and amplifier componentsfor transmission and reception of data via antenna 42. Antenna 42 may belocated within the connector block of ICD 20 or within housing ICD 20.

The various modules of ICD 20 may include any one or more processors,controllers, digital signal processors (DSPs), application specificintegrated circuits (ASICs), field-programmable gate arrays (FPGAs), orequivalent discrete or integrated circuitry, including analog circuitry,digital circuitry, or logic circuitry. Memory 40 may includecomputer-readable instructions that, when executed by control module 30or other component of ICD 20, cause one or more components of ICD 20 toperform various functions attributed to those components in thisdisclosure. Memory 40 may include any volatile, non-volatile, magnetic,optical, or electrical media, such as a random access memory (RAM),read-only memory (ROM), non-volatile RAM (NVRAM), static non-volatileRAM (SRAM), electrically-erasable programmable ROM (EEPROM), flashmemory, or any other non-transitory computer-readable storage media.

FIG. 3 is a block diagram of an example sensing channel of a sensingmodule, such as sensing module 32 of FIG. 2 or a sensing module inanother implantable medical device (e.g., a leadless pacing device). Thesensing channel may, for example, be a sensing channel for processingsensed signals on a first sensing vector. Sensing module 32 may includea similar sensing channel for each of the sensing vectors to beprocessed. In the case of multiple sensing channels, sensing module 32may include duplicate components or each sensing channel may share oneor more components.

The sensing channel illustrated in FIG. 3 includes a prefilter 50,preamplifier 52, low-pass filter 54, analog-to-digital converter (ADC)56, decimator 58, blanking module 60, pace pulse detector 62, blankingcontrol module 64, sense digital filter 66, ECG morphology digitalfilter 67, and ECG filter 68. The configuration of the sensing channelis exemplary in nature and should not be considered limiting of thetechniques a described herein. The sensing channel of sensing module 32may include more or fewer components than illustrated and described inFIG. 3.

The electrical signals sensed on a sensing vector of lead 22 areprovided to prefilter 50 of sensing module 32. The electrical signalsprovided to prefilter 50 are differential signals. Prefilter 50 mayinclude one or more passive resistor-capacitor (RC) band-pass filtersand protection diodes to filter out direct current, high frequency, andhigh voltage transient signals. The prefiltered signal from prefilter 50is provided to preamplifier 52, which amplifies the input signals by again and converts the prefiltered differential signals to a single-endedsignal.

Preamplifier 52 may, in some instances, also generate a signal when aninput or output level exceeds a range of the preamplifier (labeled“preamp over-range” in FIG. 3). The range of preamplifier may be between±10-20 millivolts (mV). However, the range may be smaller or larger inother embodiments. Preamplifier 52 may generate the preamp over-rangesignal when the input signal causes the preamplifier to be over-range.Such a condition may be indicative of an input signal greater thanapproximately 10-20 mV, which is much larger than the expected amplitudeof an electrical signal corresponding to a ventricular contraction,which would be closer to 1-5 mV. The preamp over-range signal isprovided to pace pulse detector 62 for analysis in determining whetheror not a pace spike or a pace artifact are detected as will be describedfurther below.

The preamplified signal is output by preamplifier 52 to low pass filter54. Low pass filter 54 may provide anti-alias filtering and noisereduction prior to digitization. The filtered signal output by low passfilter 54 is provided to ADC 56, which converts the analog signal to adigital bit stream. In one example, ADC 56 may be a sigma-deltaconverter (SDC), but other types of ADCs may be used. The output of ADC56 is provided to decimator 58, which functions as a digital low-passfilter that increases the resolution and reduces the sampling rate. Inone example, ADC may have an 8-bit resolution and 16 kiloHertz (kHz)sampling rate. Decimator 58 may have a 16-bit resolution and a 1 kHzsampling rate. These values are for example purposes only and should notbe considered limiting of the techniques described herein.

ADC 56 may also have other characteristics, such as an input range and aslew rate range. In one example, the input range of ADC 56 may bebetween 25-825 mV and the slew rate range may be 0 to 6.24 mV/ms, 3.12mV/ms, 1.56 mV/ms, or 0.78 mV/ms. ADC 56 may be configured to generatean ADC input over-range signal when the input signal is greater than theinput range of ADC 56. Such a condition may, for example, be indicativeof a sensed signal greater than approximately 10-20 mV peak which ismuch larger than an expected ventricular contraction 1-5 mV.Alternatively or additionally, ADC 56 may be configured to generate aslew rate over-range signal when the slew rate is faster than can betracked by ADC 56. For example, the accumulated voltage error signalinternal to ADC 56 may be monitored with a comparator and when the errorsignal exceeds the comparator threshold, the slew over-range is tripped.The slew-rate overange may, in one instance, may be generated orasserted when the slew rate of the input signal is greater than or equalto 4 mV/ms. The ADC input over-range signal and/or the slew rateover-range signal are provided to pace pulse detector 62 for analysis indetermining whether a pace spike or a pace artifact are detected.

In conventional sensing channels, the digitized signal is provideddirectly to sense filter 66 and ECG filter 68. Sense digital filter 66includes a bandpass filter (e.g., 10 to 32 Hz), rectifier, and athreshold detector. The sense digital filter 66 may, in one example,include an auto-adjusting threshold that dynamically varies between apercentage of the peak value of the signal input to sense digital filter66 and a programmed minimum value. The output of sense digital filter66, which is provided to control module 30, indicates that a cardiacevent is detected, e.g., an R-wave in the case of ventricular sensingchannel or a P-wave in the case of a atrial sensing channel, wheneverthe sensed electrical signal exceeds the threshold. In parallel with theprocessing by sense digital filter 66, diagnostic ECG filter 68 appliesa wide bandwidth filter to output an ECG signal and a morphology ECGfilter 67 applies a filter (e.g. with a bandwidth of 2.5 to 32 Hz) gooutput a signal for morphology analysis (including gross-morphologyanalysis and beat-based morphology analysis described below in furtherdetail) by control module 30.

As described above, the pacing pulses delivered by pacing device 16could interfere with sensing and tachyarrhythmia detection ofsubcutaneous ICD 20 either by decreasing sensitivity and/or specificity.FIGS. 4A and 4B illustrate example electrical signals in which pacingpulses are delivered on top of a ventricular tachycardia. FIG. 4Aillustrates an ECG of the rhythm and FIG. 4B illustrates a plotrepresenting operations occurring within sense digital filter 66. In theplot illustrated in FIG. 4B, the solid line signal is the bandpassfiltered and rectified ECG. The dotted line signal is the auto-adjustingsensing threshold of sense digital filter 66, which as described above,may dynamically vary between a percentage of the peak value of thesignal input to sense digital filter 66 and a programmed minimum value.When the ECG signal exceeds the auto-adjusting sensing threshold, asensed event is detected, as indicated by the vertical bold dashedlines. The sense digital filter outputs these detected sensed events tocontrol module 30 for further processing/analysis.

As can be seen from the illustrations of FIGS. 4A and 4B, the largeamplitude of the pacing pulses cause the auto adjusting sensingthreshold to increase to a value that is too large to detect at leastsome of the cardiac events of the underlying rhythm subsequent to thepacing pulse. In turn, control module 30 does not have an accuraterepresentation of cardiac events for use in detecting a tachyarrhythmia.The large pacing pulse may also cause artifacts in the ECG signal for ashort time after the pacing pulse due to the pacing pulse exceeding theinput range of the preamplifier, the input range of the ADC, the slewrate of the ADC, or otherwise affecting a component of the sensingchannel.

To account for the possible interference in sensing and tachyarrhythmiadetection of ICD 20 caused by the independent pacing therapy provided bypacing device 16, ICD 20 includes pace pulse detector 62, blankingmodule 60, and blanking control module 64 within the sensing channel(s).Pace pulse detector 62 obtains the signal output by ADC 56 in parallelwith decimator 58. Pace pulse detector 62 may include one or morecomponents to process the signal obtained from ADC 56 to identifycharacteristics of a pacing pulse. In one example, pace pulse detector62 may process the signal input from ADC 56 to analyze an amplitude ofthe signal, a slew rate of the signal, and/or a pulse width of thesignal. Pace pulse detector 62 may include a filter configured to passelectrical signals corresponding to pacing pulses and reject cardiacelectrical signals (e.g., a band-pass filter that passes signals havingfrequencies between approximately 100 Hz and 2000-4000 Hz, for exampleor a high-pass filter that passes signals having frequencies greaterthan 100 Hz). Alternatively or additionally, pace pulse detector 62 mayinclude a differentiator, difference filter, or a first order derivativefilter that may be used to obtain a signal representative of the slewrate of the sensed signal.

Pace pulse detector 62 may also include one or more threshold detectors.For example, pace pulse detector may include a slew rate thresholddetector that compares the output of a differentiator or a first orderderivative filter to a slew rate threshold. If the slew rate exceeds theslew rate threshold, pace pulse detector 62 determines that the signalcorresponds to a pacing pulse. Pace pulse detector 62 may likewiseanalyze the amplitude of the input signal. In some instances, pace pulsedetector 62 may analyze a combination of slew rate and amplitude todetect the presence of a pacing pulse. For example, if the slew rateexceeds the slew rate threshold, pace pulse detector 62 may compare theamplitude of the sensed signal to one or more amplitude thresholds usingamplitude threshold detectors.

In some instances, pace pulse detector 62 may include a plurality ofpace pulse detectors. In one embodiment, pace pulse detector 62 mayinclude two pace pulse detectors. A first detector, e.g., referred toherein as a pace artifact detector, has a first threshold that isconfigured to detect only pacing pulses having characteristics, e.g.,large enough in amplitude, slew rate, and/or pulse width, to impact thesensitivity for tachyarrhythmia detection of ICD 20. Such pacing pulseswill be referred to herein as pace artifacts. In one example, the paceartifact detector may be configured to detect pacing pulses havingamplitudes that are greater than or equal to 2-10 mV for pulse widths ofapproximately 1 ms. In another example, the pace artifact detector mayconfigured to detect pacing pulses having amplitudes that are greaterthan or equal to 4 mV and pulse widths of approximately 1 ms. However,the characteristics of the pacing pulses that the pace artifact detectoris configured to detect may be different.

A second detector, e.g., referred to herein as a pace spike detector,has a second threshold that is configured to detect all pacing pulsesregardless of whether they are large enough or have othercharacteristics to impact tachyarrhythmia detection. These pacing pulseswill be referred to herein as pace spikes. In one example, the pacespike detector may be configured to detect pacing pulses havingamplitudes that are greater than or equal to 1 mV and pulse widths ofapproximately 1 ms. However, the characteristics of the pacing pulsesthat the pace spike detector is configured to detect may be different.Although no modifications of the electrical signal will occur for thesesmaller pacing spikes, control module 30 may still utilize thisinformation in its tachyarrhythmia detection. The pace spike detectorwill have a higher sensitivity than the pace artifact detector so thatit can detect pacing pulses having small amplitudes and/or pulse widths.As such, pace artifacts will also be detected as pace spikes. As such,pace pulse detector 62 may analyze the slew rate, amplitude, pulse widthand/or other characteristic to detect pace artifacts and pace spikes.

In further instances, pace pulse detector 62 may include only a singledetector or more than two pulse detectors. For example, pace pulsedetector 62 may include a third pulse detector to detect noise ordetermine margin between a peak of the pacing pulse signal (e.g., spikeor artifact) and the threshold of one or both of the first and secondpulse detectors. An example pace pulse detector 62′ that includes threedetectors is described below with respect to FIG. 8.

In addition to inputting the signal from ADC 56, pace pulse detector 62also obtains the preamp over-range signal from preamplifier 52, the ADCinput over-range signal from ADC 56, and the slew rate over-range signalfrom ADC 56. All or at least some of these signals may be indicative ofa pacing artifact. For example, a preamplifier over-range signal that ispresent or asserted for a threshold period of time is likely indicativeof a sensed signal that is much larger than an expected ventricularcontraction 1-5 mV. As another example, an ADC slew rate over-rangesignal that is present or asserted for more than a threshold amount oftime, e.g., approximately 1 ms, is likely indicative of a pacingartifact as the slew rate limit of ADC 56 would not be exceeded for avery long time for EMI (e.g., less than 1 ms) and never exceeded forsensed ventricular contractions. In some instances, the threshold timemay be adjustable. In a further example, an ADC input over-range signalthat is present or asserted for more than a threshold amount of time,e.g., about 1 ms, is likely indicative of a sensed signal that is has ahigh amplitude for much longer than a ventricular contraction. As such,each of these over-range signals may meet particular criteria that islikely indicative of the presence of a pace pulse that is high enough inamplitude and/or pulse width to impact the sensitivity fortachyarrhythmia detection by ICD 20, i.e., a pace artifact. Thesecriteria will be referred to as over-range conditions. In otherexamples, the simple fact the over-range condition occurs (regardless ofhow long it occurs for) may be an over-range condition.

Pace pulse detector 62 analyzes these over-range signals as well as thepace spike analysis and/or pace artifact analysis performed as describedabove and outputs a pace artifact detection signal and a pace spikedetection signal based on the analyses. In one example, pace pulsedetector 62 generates and/or asserts the pace artifact detect signalwhen any of the over-range conditions are met or the amplitude, slewrate, and pulse width analysis indicates that the presences of a pacingartifact. Likewise, pace pulse detector 62 generates and/or asserts thepace spike detect signal when any of the overrange conditions are met orthe amplitude, slew rate, and pulse width analysis indicates that thepresences of a pacing spike. The pace artifact analysis and the pacespike analysis may be capable of detecting pace artifacts and pacespikes that are not large enough to trigger the over-range conditionsdescribed above. Pace pulse detector 62 outputs the pace artifact detectsignal to blanking control module 64 and outputs the pace spike detectsignal to control module 30.

Blanking control module 64 initiates removal of the pulse from theelectrical signal when the pace artifact detect signal is asserted.Although described herein as removing the pulse from the electricalsignal, the pulse may not be completely removed from the electricalsignal, but its effect on sensing accuracy is essentially neutralized.For example, the electrical signal may be modified such that the pulseis not mistaken for an intrinsic cardiac event (e.g., a ventricularevent). As such, blanking control module 64 initiates removal of thepulse when any one of the over-range conditions is met or the paceartifact detection analysis indicates the presence of a pacing pulsethat is high enough in amplitude and/or pulse width to impact thesensitivity for tachyarrhythmia detection. The modifications to thesensing channel to remove the pulse may continue for a predeterminedperiod of time, until the pace artifact detect signal is deasserted, oruntil the pace artifact signal has been deasserted for a certain periodof time. In some instances, blanking control module 64 may be configuredto delay the initiation of the pulse removal to account for any delay ofthe signal through decimator 58 or other components of the sensingchannel. The delay may be programmable and may have a value between ≥1and ≤60 milliseconds (ms), and more preferably ≥5 and ≤25 ms. This mayreduce the overall duration of the sensing channel modificationsimplemented to remove the pulse. In some instances, blanking controlmodule 64 may use information regarding blanking done for a previouspulse to determine the length of blanking, to build confidence ifblanking for current detected pacing pulse is appropriate, or the like.

In one example, blanking control module 64 may initiate blanking on onlythe sensing channel on which the pacing artifact was detected. Inanother example, blanking control module 64 may initiate blanking on allof the sensing channels when a pacing artifact is detected on any one ofthe sensing channels. For example, blanking control module 64 may causea second blanking module in a second sensing channel to blank as well asblanking module 60 in the first sensing channel. When blanking isdesired, blanking control module 64 provides a control signal toblanking module 60 to initiate blanking of the signal output fromdecimator 58. Blanking module 60 may, in one example, include a sampleand hold circuit that holds the value of the signal at a current valuein response to receiving the control signal from blanking control module64. The current value may be a value of the electrical signal prior tothe detected pulse. Blanking module 60 continues to hold the value ofthe sensed electrical signal until the blanking control module 64removes or deasserts the control signal. In one example, blankingcontrol module 64 may apply the control signal or hold signal, and thuscause blanking, for less than or equal to approximately forty (40) ms.In another example, blanking control module 64 may apply the hold signalfor less than or equal to approximately thirty (30) ms. In anotherexample, blanking control module 64 may apply the hold signal for lessthan or equal to approximately twenty (20) ms. In other embodiments,blanking module 60 may include an interpolation module that provides alinear interpolation or other interpolation between a first value at thetime the control signal is initiated or asserted (e.g., prior to thedetected pulse) and a second value at the time the control signal isremoved or deasserted (e.g., subsequent the detected pulse). This periodof time may be considered a blanking period as the electrical signal isessentially blanked to remove the pulse.

Blanking module 60 may, in some instances, also include a delay blockthat introduces a delay into the electrical signal prior to the sampleand hold circuit to allow for detection of the pacing pulse by pacepulse detector 62 and analysis of the inputs by blanking control module64 to determine whether to blank the electrical signal before theartifact from the pacing pulse has a chance to propagate into the senseand ECG outputs. The delay introduced into the sensing channel may bebetween approximately 1-20 ms depending up on where in the sensingchannel the blanking occurs and whether or not blanking module 60performs interpolation as described above. In some instances, this delayblock may not exist or may be for a shorter period of time since thedecimator 58 also provides some delay between the ADC output and theblanking module 60.

In other instances, pace pulse detector 62 may process and detect thepacing artifact faster than the time it takes the signal to propagatefrom ADC 56 to pulse removal module 60. In this case, pulse removalcontrol module 64 may delay application of the signal that causes pulseremoval module 60 to hold and/or interpolate the sensed signal toaccount for any delay of the sensed signal through decimator 58 and/orother component of the sensing channel. This may reduce the overallduration of the blanking time resulting in a more accurate sensedsignal.

The output of blanking module 60 is provided to sense digital filter 66,ECG morphology filter 67, and diagnostic digital ECG filter 68, whoseoperation are described above. By providing the blanking describedabove, the pace artifact is significantly reduced as illustrated in theplots in FIG. 5A and FIG. 5B. FIG. 5A illustrates the same signal asFIG. 4A, but after the signal is modified to remove pacing pulses. Themodification in the example of FIG. 5A includes a 24 ms blanking appliedto each of the detected pace artifacts. Likewise, FIG. 5B illustratesthe plot of operations within the digital sense filter 66. As can beseen in FIG. 5B, by blanking the sensing channel in response todetecting pace artifacts, the auto-adjusting threshold remains in a zonethat is capable of detecting all of the cardiac events, thus reducingthe likelihood of undersensing. Moreover, digital sense filter 66 doesnot falsely detect the pace spikes as intrinsic R-waves, thus reducingthe likelihood of oversensing. The techniques of this disclosuretherefore provide control module 30 with more accurate sensinginformation to monitor for tachyarrhythmia.

The sensing channel illustrated in FIG. 3 is one example sense channel.Other configurations of a sense channel or arrangement of components inthe sense channel may be utilized without departing from the scope ofthis disclosure. In other embodiments, for example, pace pulse detector62 may obtain its input from other components earlier in the sensingchannel processing stage, e.g., from prefilter 50, preamplifier 52, orlow-pass filter 54. In another example, blanking module 60 may belocated elsewhere within the sensing channel, such as betweenpreamplifier 52 and low-pass filter 54. In such an example, the blankingmay be implemented using a resister in series with a switch to create asample and hold circuit.

FIG. 6 is block diagram illustrating an example pace pulse detector 62.Pace pulse detector 62 includes a filter 90, a derivative (dV/dt) filter91, a rectifier 92, a pace artifact detector 94, and a pace spikedetector 96. Pace pulse detector 62 inputs the signal output by ADC 56.This signal is provided to filter 91, dV/dt filter 91, pace artifactdetector 94 and pace spike detector 96. However, the various componentsof pace pulse detector 62 may obtain the signal from other components ofthe sensing channel, such as directly from the preamplifier 52.

Filter 90 of pace pulse detector 62 filters the signal output from ADC56. Filter 90 may be configured to pass electrical signals correspondingto pacing pulses and reject cardiac electrical signals. Filter 90 may,in one example, be a band-pass filter that passes signals havingfrequencies between approximately 100 Hz and 1000-4000 Hz. In anotherexample, filter 90 may be a high-pass filter that passes signals havingfrequencies greater than 100 Hz. In other examples, filter 90 may beanother type of filter, such as a derivative filter. In a furtherexample, the signal may not be filtered at all. Rectifier 92 rectifiesthe filtered signal from filter 90. The rectified signal is then isprovided to pace artifact detector 94 and pace spike detect detector 96.

The dV/dt filter 91 generates a difference signal (e.g., x(n)-x(n−1)) ofthe output of ADC 56. The difference signal includes spikes thatcorrespond with portions of the signal having high slew rates. Thedifference signal is also provided to pace artifact detector 94 and pacespike detect detector 96.

Pace artifact detector 94 and pace spike detector 96 analyze some or allof the raw input signal from ADC 56, the rectified signal from rectifier92, the difference signal from dV/dt filter 91 to detect the presence ofa pace artifact and a pace spike, respectively. In one example, paceartifact detector 94 and pace spike detect detector 96 may detect thepace artifact and pace spike, respectively, using only amplitude or onlyslew rate. In another example, pace artifact detector 94 and pace spikedetect detector 96 may detect the pace artifact and pace spike,respectively, using a combination of amplitude, slew rate, and pulsewidth. Depending on the type of analysis performed, pace pulse detector62 may not include some of the components illustrated (e.g., filter 90,dv/dt filter 91, and/or rectifier 92). For example, if pace detector 94and 96 do not analyze slew rate, detector 62 may not include dv/dtfilter 91. However, in other embodiments, pace pulse detector 62 mayinclude all the components and be configurable to analyze differentaspects of the sensed signal.

Pace artifact detector 94 and pace spike detect detector 96 may compareraw input signal from ADC 56, the rectified signal from rectifier 92,the difference signal from dV/dt filter 91 to respective thresholds todetect the pace artifact and/or the pace spike. The thresholds of paceartifact detector 94 and pace spike detector 96 may be different suchthat the pace artifact detector 94 is configured to only detect paceartifacts having large enough amplitudes to impact the tachyarrhythmiadetection algorithm performed by control module 30 while pace spikedetector 94 is configured to detect pacing pulses regardless of whetherthey are large enough to impact the tachyarrhythmia detection algorithmperformed by control module 30. Therefore, the pace artifactthreshold(s) (e.g., artifact slew rate threshold or amplitude threshold)therefore are generally larger than the pace spike threshold(s) (e.g.,spike slew rate threshold or amplitude threshold). As such, pace spikedetector 94 will have a higher sensitivity than the pace artifactdetector 96 so that it can detect pacing pulses with smaller amplitudesand pulse widths. In one example, the thresholds of pace pulse detector62 may be set such that pace artifact detector 94 is configured todetect pacing pulses are greater than or equal to 2-10 mV at pulsewidths of approximately 1 ms and pace spike detector 96 is configured todetect pacing pulses are greater than or equal to 1 mV at pulse widthsof approximately 1 ms. However, the thresholds may be configurableand/or be configured to detect pacing pacing spikes and artifacts havingdifferent characteristics.

In some instances, some or all of the pace artifact thresholds and thepace spike thresholds may be automatically adjustable. For example, oneor both of pace artifact amplitude threshold and the pace spikeamplitude threshold may be dynamically adjusted based on the peakamplitude of the detected pulse to allow threshold to be raised higherto avoid EMI if the detected pace pulses are large in amplitude.Alternatively or additionally, one or both of the pace artifactamplitude threshold and the pace spike amplitude threshold may bedynamically adjusted based on a baseline R-wave amplitude. In this case,if the R-waves are large, the threshold for sensing pace artifactsand/or pace spikes may need to set higher. In one example, the increasemay be proportionate, e.g., a 50% increase in sensed R-wave amplitudewould lead to a 50% increase in pacing artifact detection threshold.

As further illustrated in FIG. 6, pace artifact detector 94 and pacespike detector 96 also receive the over-range signals from the variouscomponents of the sensing channel (e.g., the preamp over-range signalfrom preamplifier 52, the ADC input over-range signal from ADC 56, andthe slew rate over-range signal from ADC 56). Based on the analysis ofthe over-range signals and the processing of the signals output by ADC56, pace artifact detector 94 and pace spike detector 96 output a paceartifact detect signal and a pace spike detect signal, respectively. Inone example, pace artifact detector 94 generates and/or asserts the paceartifact detect signal when any of the over-range conditions are met aremet or the amplitude, slew rate, and/or pulse width analysis of the ADCoutput indicates the presence of a pacing artifact. Likewise, pace spikedetector 96 generates and/or asserts the pace spike detect signal whenany of the over-range conditions are met or the amplitude, slew rate,and/or pulse width analysis indicates the presences of a pacing spike.

The pace artifact detect signal is provided to blanking control module64 to initiate blanking of one or more of the sensing channels,described in further detail below. Because blanking of the sensingchannel(s) may introduce an artifact in the ECG signal, it is desiredthat blanking is only done when necessary to obtain good tachyarrhythmiadetection sensitivity, thus the higher pace artifact thresholds.

The pace spike detect signal and, in some instances, the pace artifactdetect signal, may be provided to control module 30 to be used as partof the tachyarrhythmia detection. The pace artifact detect signal andthe pace spike detect signal may be provided directly to control module30 by pace pulse detector 62 or relayed to control module via blankingcontrol module 64. The pace artifact signal and the pace spike detectsignal may be provided individually to control module 30. Alternatively,the pace artifact detect signal and the pace spike detect signal couldbe logically combined (e.g., logically OR'ed) and provided to controlmodule 30. In instances in which multiple sensing channels are analyzed,the pace artifact signal and the pace spike detect signal for each ofthe sensing channels may be provided individually or logically combined.

The pace artifact detect signal and the pace spike detect signal may beprovided to control module 30 using any of a number of techniques. Forexample, the pace artifact detect signal and the pace spike detectsignal outputs from one or both of the sensing channels could belogically combined to generate a single output and used to generate aninterrupt signal to control module 30. The advantage of combiningsignals and generating an interrupt is that it provides notification ofthe pacing event in a very short time allowing the control module 30 toquickly respond to a pacing pulse. The drawback is that it is possiblethat an excessive number of interrupts could be generated in certainconditions which may overload the ability of control module 30 to handlethe interrupts or cause excessive current drain. Alternatively, the paceartifact detect signal and the pace spike detect signal from all activechannels could be combined into a single register and continuouslystreamed over to control module 30 for storage in memory and lateranalysis. This provides the advantage of providing more informationabout the amplitude of the pacing pulse and which channel the pulse wasdetected on. It also allows control module 30 to process the pacinginformation on a regular schedule or when processing data fortachyarrhythmia detection rather than as an interrupt which reduceconcerns with over-burdening control module 30 with interrupt handling.The drawback to this approach is that it requires additional memory andincreases the latency from the pacing pulse being detected until controlmodule 30 can act on the information. In some instances, additionalinformation may be sent along with the pace pulse detection data. Forexample, the signal to control module 30 may also specify where the pacepulse detect occurs within the V-V interval. In some instances, ICD 20or control module 30 may include an event queue that provides some orall of the information from sensing module 32 to control module 30. Oneexample of such an event queue is described in U.S. Pat. No. 8,855,780(Hansen et al.) entitled, “PACEMAKER EVENT QUEUE TO CONTROL DEVICEPROCESSOR OPERATING POWER,” the contents of which is incorporated hereinin their entirety.

Pace pulse detector 62 of FIG. 6 is one example of such a detector. Inother embodiments, pace pulse detector 62 may include only a singledetector instead of a pace artifact detector 94 and pace spike detector96. In further embodiments, pace pulse detector 62 may include more thantwo pulse detectors. For example, pace pulse detector 62 may include athird pulse detector to detect noise or determine margin between a peakof the pacing signal and the respective threshold of pace artifactdetector 94 or pace spike detector 96. This is described further withrespect to FIG. 8. However, noise and/or signal margin may be detectedusing other techniques. For example, pace pulse detector 62 may includea peak detector configured to measure the peak of the detected pacingpulse and the peak may be used to determine whether pace artifactdetector 94 and pace spike detector 96 have adequate margin for reliabledetection of pacing pulses.

FIG. 7 is a conceptual diagram illustrating example operation of pacepulse detector 62. FIG. 7 illustrates an example sensed electricalsignal that includes a pacing train 70 that includes at least threepacing pulses 72. FIG. 7 also illustrates an example a slew rate signal74, which may be output by filter 90 (e.g., a difference filter or firstorder derivative filter) of pace pulse detector 62. As illustrated inFIG. 7, slew rate signal 74 has spikes 76 that correspond with the edgesof pacing pulses 72. Pace pulse detector 62 may compare slew rate signal74 to a slew rate threshold 78 and when slew rate signal 74 exceeds theslew rate threshold, pace pulse detector 62 may detect the presence of apacing spike. In order to avoid detecting the trailing edge of pacingpulse 72 as separate pace pulse, pace pulse detector 62 may not countany spike 76 that occurs within a particular period of time, e.g., 2 ms,from a previous spike 76 as a separate pacing pulse. In some instances,pace pulse detector 62 may, however, track these close proximity spikesto estimate pulse width of the pacing pulses. In other examples,detecting a slew rate that exceeds the slew rate threshold would resultin further analysis of other characteristics of the detected signal,such as looking at the amplitude of the sensed electrical signal. In oneinstance, the example slew rate threshold may be equal to 4 mV/ms.However, other thresholds may be utilized.

FIG. 8 is a block diagram illustrating another example pace pulsedetector 62′. Pace pulse detector 62′ includes a decimator 180, CICfilter 182, moving average filter 184, bandpass filter 186, a derivative(dV/dt) filter 188, threshold detectors 190A-190C, and noise detectors192A-192B. Pace pulse detector 62′ may be utilized in a sensing channel,such as the sensing channel of FIG. 3. In other words, pace pulsedetector 62′ may be used in place of pace pulse detector 62.

Pace pulse detector 62′ inputs the signal output by ADC 56. Pace pulsedetector 62′ makes use of decimation in order to attenuate highfrequency noise and to allow the use of shorter filters at lowerfrequencies, thereby saving power. Decimator 180 decimates the digitizeddata output from ADC 56. In one example, decimator 180 may decimate thesignal by a decimation factor of 2. For instance, ADC 56 may output an8-bit signal at 32 kHz and decimator may reduce the sampling rate of thesignal to output a 16 kHz signal. However, decimator 180 may performdecimation using other decimation factors.

Cascaded integrator-comb (CIC) filter 182 inputs the signal output bydecimator 180. CIC filter 182 attenuates frequencies above a particularfrequency band and further decimates the signal output by decimator 180.In one example, CIC filter 182 is a second-order CIC filter. In oneexample, CIC filter 182 may input a 16 kHz signal output by decimator180 and decimate the signal to 4 kHz and increase resolution to 14 bits.CIC filter 182 may also attenuate frequencies above 4 kHz. In anotherexample, the decimation function can be combined with the CIC filter todecimate directly from 32 Khz/8 bit down to 4 Khz/14 bit without using aseparate decimate function. Other decimation and resolution changes maybe used without departing from the scope of this disclosure.

Moving average filter 184 inputs the signal output by CIC filter 182.Moving average filter 184 averages a number of samples from the signaloutput by CIC filter 182 to produce each sample of the output signal. Inthis manner, moving average filter 184 may provide furtherhigh-frequency attenuation. In one example, moving average filter may bea second order moving average filter, but other moving average filtersmay be utilized.

Bandpass filter 186 inputs the signal output by moving average filter184. Bandpass filter 186 passes frequencies within a certain range andrejects (or attenuates) frequencies outside of that range. In oneexample, bandpass filter 186 may be a 200 Hz bandpass filter. The outputof bandpass filter 186 is provided concurrently to threshold detectors190A-C.

Threshold detectors 190A-190C each compare the signal output frombandpass filter 186 to a respective amplitude threshold and output asignal (e.g., DETECT1, DETECT2, DETECT3) based on the comparison. Forexample, when the signal output of bandbass filter 186 exceeds therespective amplitude threshold, the detect signal of the respectivethreshold detector 190 may go active for a period of time. Thethresholds of each of threshold detectors 190 may be different from oneanother as explained in more detail below.

For instance, threshold detector 190A may have a first threshold,threshold detector 190B may have a second threshold that is smaller thanthe first threshold, and threshold detector 190C may have a thirdthreshold that is smaller than both the first and second thresholds. Thefirst threshold of threshold detector 190A may be set to a level todetect pace pulses having large enough amplitudes to be detected asintrinsic R-waves, VT, or VF by sense digital filter 66 and/or to impactthe tachyarrhythmia detection algorithm performed by control module 30(e.g., pace artifacts). In one example, the threshold of thresholddetector 190A may be set such that threshold detector 190A detectspacing pulses having amplitudes that are greater than or equal to 2-10mV for pulse widths of approximately 1 ms, similar to the pace artifactdetector 94 of FIG. 6. In another example, the threshold of thresholddetector 190A may be set such that threshold detector 190A detectspacing pulses having amplitudes that are greater than or equal to 4 mVfor pulse widths of approximately 1 ms. However, the characteristics ofthe pacing pulses that the pace artifact detector is configured todetect may be different.

The threshold of threshold detector 190B may be set to a level at whichthreshold detector 190B detects pace pulses regardless of whether thepace pulse would be detected as an intrinsic R-wave and/or impact thetachyarrhythmia detection algorithm (e.g., pace spikes). As describedabove, the threshold of detector 190B is less than the threshold ofdetector 190A. In one example, the threshold of threshold detector 190Bmay be set such that threshold detector 190B detects pacing pulseshaving amplitudes that are greater than or equal to 1 mV and pulsewidths of approximately 1 ms, similar to the pace spike detector 96 ofFIG. 6.

The threshold of threshold detector 190C may be set to a level at whichthreshold detector 190C detects noise. As described in the exampleabove, the threshold of detector 190C is less than the threshold ofdetector 190A and 190B. In this case, detector 190C detects signalswhich are lower in amplitude than would be detected by either 190A or190B. As an example, threshold for detector 190C may be set a fractionor percentage of the threshold of detector 190B, e.g., 0.5X-0.75X where“X” is the threshold of detector 190B. These lower amplitude pulses maybe interpreted as noise if they occur at higher frequencies or atdifferent points in the cardiac cycle than expected for pacing pulses.Thus, threshold detector 190C can be set to detect EMI noise andartifacts, which might have different characteristics than paceartifacts/spikes, e.g., lower amplitude, different slew rate, anddifferent frequency (e.g., rate at which pulses occur). In someinstances, the threshold of detector 190C may be periodically adjustedto different levels to estimate the noise floor to enable thresholds190A and 190B to be set to a level above the noise floor determined byusing detector 190C.

In an alternative embodiment, threshold detector 190A may have a firstthreshold, threshold detector 190B may have a second threshold that issmaller than the first threshold, and threshold detector 190C may have athird threshold that is larger than either one or both the first andsecond thresholds. The first and second thresholds may be set at levelssuch that the first threshold detector 190A detects pace pulses (e.g.,artifacts) having large enough amplitudes to be detected as intrinsicR-waves by sense digital filter 66 or to impact the tachyarrhythmiadetection algorithm performed by control module 30 and the secondthreshold detector 190B detects pace pulses (e.g., spikes) regardless ofwhether the pace pulse would be detected as an intrinsic R-wave and/orimpact the tachyarrhythmia detection algorithm. Example threshold valuesare described in the example above.

In this embodiment, however, the threshold of threshold detector 190Cmay be set to a level at which threshold detector 190C detects a largersignal amplitude than either threshold detector 190A or thresholddetector 190B. In this manner, threshold detector 190C may be used todetermine whether the thresholds of detectors 190A and/or 190B havesufficient margin. As described above, the threshold of detector 190Cmay be greater than either or both of the threshold of detector 190A and190B. In one example, the threshold of detector 190C may be set to alevel at which threshold detector 190C detects pulses that are a factorlarger than detected by detector 190B, e.g., 1.5X-2X where “X” is thethreshold of detector 190B. If it is determined that detector 190C isunable to detect pacing spikes detected by 190B, pace detector 62′ maydetermine that detector 190B has inadequate margin. In an alternativeembodiment, pace detector 62′ may include peak detector instead of thirdthreshold detector 190C that may be used to determine whether thresholddetectors 190A and/or 190B have adequate margins. If adequate margindoes not exist, one or more of the detect signals (DETECT1, DETECT2,DETECT3) may be ignored or disregarded. Alternatively, pace detector 62′may automatically adjust the threshold of detector 190B and/or 190A to alower level in response to determining that there is inadequate margin.

In a further embodiment, detector 190C can be used both for verificationof sufficient threshold to noise margin by using a threshold of, e.g.,0.5X-0.75X of threshold 190B, and for verification of signal tothreshold margin by using a threshold of, e.g., 1.5X-2X of threshold190B. This information can be used to adjust the threshold of 190B overtime to provide both adequate threshold to noise ratio and signal tothreshold ratio.

In some instances, pace detector 62′ may include a plurality of quiettimers 194 that may inhibit their respective threshold detectors fromindicating more than one pace detection per pace pulse. In one example,each threshold detector 190 may have a respective quiet timer 194. Inanother example, one or more of the threshold detectors 190 may have acommon quiet timer 194. The outputs of quiet timers 194 goes active whenthe signal of a respective one of the threshold detectors 190 exceedsthe programmed threshold. When one or more of the quiet timers 194 goactive, the threshold detectors 190A-190C associated with those quiettimers do not detect a further pace pulse. In this manner, quiet timers194 block further pace detects until the quiet timers 194 deactivatestheir outputs. In one example, quiet timers 194 may activate theiroutputs for ≥10 ms and ≤40 ms upon the threshold detector 190 associatedwith the quiet timer 194 outputs an active detect signal. In anotherexample, quiet timers 194 may activate their outputs for 30 ms. In someinstances, quiet timers 194 may operate in retrigger mode (aprogrammable feature), which restarts quiet timers 194 if anotherdetection occurs while the output of quiet timer 194 is active. In otherinstances, pace pulse detector 62′ does not include quiet timer 194.

In the example illustrated in FIG. 8, derivative filter 188 also obtainsthe signal output from moving average filter 184. Derivative filter 188and noise detectors 192 process the sensed electrical signal in parallelwith bandpass filter 186 and threshold detectors 190. Derivative filter188 may be a first order difference derivative filter. For example,derivative filter may generate a difference signal (e.g., x(n)−x(n−1))of the output of moving average filter 184. In other embodiments otherderivative filters may be utilized.

In some embodiments, pace pulse detector 62′ includes noise detectors192 analyze the output of derivative filter 188 to detect noise signals.In other instances, no noise detection is performed by pace pulsedetector 62′. In instances in which pace pulse detector 62′ does includenoise detectors 192, each of noise detectors 192 may monitor for a noiseof a particular frequency, e.g., noise detector 192A may monitor for 60Hz noise and noise detector 192B may monitor for 50 Hz noise. Pace pulsedetector 62′ may, in other embodiments, include only a single noisedetector or more than two noise detectors.

In the example described above in which the output of the moving averagefilter is at 4 kHz, each of noise detectors 192 may include azero-crossing counter that counts the number of 4 kHz cycles betweenzero crossings. Alternatively, there may be only a single zero-crossingcounter that is shared by the noise detectors 192 and that tracks thenumber of 4 kHz cycles between zero crossings. Each of noise detectors192 also includes a noise counter that is incremented and decremented asa function of the value of the zero-crossing counter when azero-crossing occurs. Noise detectors 192 detect noise based on thevalue of the noise counter. In the case of noise detector 192Amonitoring for 60 Hz noise, upon occurrence of a zero crossing, noisedetector 192A determines the value of the zero-crossing counter andincrements the 60 Hz noise counter if the value of the zero-crossingcounter is within a range indicative of 60 Hz noise, e.g., is between 32and 36, and decrements the 60 Hz noise counter if the value of thezero-crossing counter is not within the range indicative of 60 Hz noise,e.g., less than 32 or greater than 36. For 60 Hz noise, which has aperiod of 16.67 ms, it would be expected that a zero crossing occursapproximately every 8.33 ms. A 4 kHz clock cycle is 0.25 ms, so 8.33 msis equal to a count of 33.3 clock cycles between zero crossings. Therange is builds in some variance. Likewise, in the case of noisedetector 192B monitoring for 50 Hz noise, upon occurrence of a zerocrossing, noise detector 192B determines the value of the zero-crossingcounter and increments the 50 Hz noise counter if the value of thezero-crossing counter is within a range indicative of 50 Hz noise, e.g.,between 39 and 43 and decrements the 50 Hz noise counter if the value ofthe zero-crossing counter is not within the range indicative of 50 Hznoise, e.g., less than 39 or greater than 43. The range is selected in amanner similar to the 60 Hz noise range using the period of a 50 Hzsignal in place of the 60 Hz noise signal. If either of the 60 Hz noisecounter or the 50 Hz noise counter is greater than or equal tothreshold, e.g., 7 in one example, the respective noise detect signal(e.g., NOISE DETECT1 or NOISE DETECT2, respectively) goes active. Theparticular noise detect signal remains active until the respective noisecounter falls below a second threshold value, e.g., 4 in one example, atwhich point the active noise detects signal goes to inactive. Having thesecond threshold value be lower than the first threshold value providesa buffer so that the noise detector does not toggle between detectingnoise and not detecting noise as frequently. Control module 30 mayutilize the outputs of noise detectors 192A and/or 192B to determine ifa pace detection is a false detection due to noise.

In other instances, the zero-crossing counter may count the number ofdata samples between each zero crossing and the thresholds could be setaccordingly. In another alternative, timestamps of the zero crossingscould be utilized to determine the frequency of the signals. In yetanother instances, the noise detector may detect other noise orartifacts besides 50 and 60 Hz noise.

FIG. 9 is a flow diagram illustrating example operation of blanking ofone or more sensing channels in accordance with the techniques describedherein. Initially, pace pulse detector 62 obtains and analyzes one ormore inputs associated with detection of a pacing pulse in the sensingchannel (80). In the example sensing module 30 of FIG. 3, for example,pace pulse detector 62 analyzes some or all of a pace artifact detectsignal (e.g., based on slew rate, amplitude, pulse width or othercharacteristic of the received signal), preamp over-range signal, ADCinput over-range signal, and ADC slew rate over-range signal. However,in other embodiments, only one of these signals or any combination oftwo or more of these signals may be analyzed by pace pulse detector 62.Additionally, other signals indicative of a pacing pulse or otherartifact in the sensing channel may be analyzed by pace pulse detector62. Different approaches using a single input or multiple inputs willresult in different tradeoffs between sensitivity, specificity,complexity. In some instances, pace pulse detector 62 attempts to limitthe blanking of the sensing channel to situations in which the pacepulse is likely to affect tachyarrhythmia detection sensitivity orspecificity, e.g., higher amplitude pace pulses or pace artifacts.

Pace pulse detector 62 determines whether any of the inputs areindicative of a pacing pulse requiring blanking, i.e., a pacing artifact(82). As described above with respect to FIG. 3, a pace pulse may havean amplitude, slew rate, or other characteristic that is different thansensed signals (e.g., sensed R-waves or P-waves). For example, pacepulses having amplitudes of greater than approximately 10-20 mV mayresult in preamplifier 52 and/or ADC 56 to operate in one or more of theinput over-range conditions. As another example, pace pulses may haveslew rates that exceed the slew rate limit of ADC 56, thus causingactivation of the ADC slew rate over-range signal. Likewise, the paceartifact detector 94 may detect a pace pulse likely to cause an artifactbased on the amplitude, slew rate, or other characteristic of the signalfrom ADC 56 or other component. When none of the input signals isindicative of a pacing pulse requiring blanking (“NO” branch of block82), blanking control module 64 continues to analyze the one or moreinputs (80).

When any one of the input signals is indicative of a pacing pulserequiring blanking (“YES” branch of block 82), pace pulse detector 62asserts the pace artifact detect signal (83). In response to theassertion of the pace artifact detect signal, Blanking control module 64determines whether the sensing channel has been blanked within athreshold period of time (84). In one example, blanking control 64 willnot blank the sensing channel until a period of at least 30-60 ms haspassed since the last time the sensing channel was previously blanked.This is intended to prevent excessive blanking in a continuous EMIenvironment, but still allow blanking on both atrial and ventricularpaced events at intervals less than approximately 200 ms. When blankinghas been triggered within the threshold period of time (“YES” branch ofblock 84), blanking control module 64 will not blank the sensing channeland will continue to analyze the one or more inputs (80).

When blanking has not been triggered within the threshold period of time(“NO” branch of block 84), blanking control module 64 initiates blankingof the sensing channel (86). In one example, blanking control module 64may initiate the blanking of the sensing channel by providing a controlsignal to blanking module 60 to cause the blanking module to hold thevalue of the sensed signal, as described above with respect to FIG. 3.In one example, blanking control module 64 may initiate blanking on onlythe sensing channel on which the pacing artifact was detected. Inanother example, blanking control module 64 may initiate blanking on allof the sensing channels when a pacing artifact is detected on any one ofthe sensing channels.

After initiating the blanking of the sense channel, blanking controlmodule 64 determines whether the amount of time that the channel hasbeen blanked is greater than a blanking threshold (88). In someinstances, blanking control module 64 may be configured to blank for apredetermined period of time, e.g., 20 ms. When the sensing channel hasnot been blanked for the predetermined amount of time, (“NO” branch ofblock 88), blanking control module 64 continues to blank the sensingchannel. When the sensing channel has been blanked for the predeterminedamount of time, (“YES” branch of block 88), blanking control module 64discontinues the blanking of the sensing channel (89).

In another embodiment, blanking control module 64 may not blank thesensing channel for a predetermined period of time. Instead, blankingcontrol module 64 may continue to blank the sensing channel until all ofthe inputs no longer indicate presence of a pacing pulse requiringblanking, all of the inputs no longer indicate presence of a pacingpulse for a threshold period of time, e.g., 5-20 ms, allowing forsensing channel components to settle, or the amount of time sinceinitiating the blanking of the sensing channel is greater than or equalto a maximum blanking duration, e.g., approximately 10-30 ms.

FIG. 10 is a state diagram 100 of an example tachyarrhythmia detectionalgorithm. During normal operation, ICD 20 operates in a not concernedstate 102 in which control module 30 estimates the heart rate of thesensed electrical signals on one or more sensing channels. Controlmodule 30 of ICD 20 may measure a plurality of R-R intervals (i.e.,intervals between consecutive sensed ventricular events) on the sensingchannel and estimate the heart rate of the sensing channel based on theplurality of measured R-R intervals. In one example, control module 30stores the most recent 12 R-R intervals on the sensing channel. However,control module 30 may store more or fewer than the 12 most recent R-Rintervals. To estimate the heart rate, control module 30 may sort thestored R-R intervals from shortest to longest R-R intervals and estimatethe heart rate using only a subset of the R-R intervals. In one example,control module 30 may estimate the heart rate as an average of a subsetof the measured R-R intervals (e.g., the average of the 7th through 10thshortest R-R intervals of the most recent 12 R-R intervals). More orfewer R-R intervals may be used in the estimation of the heart rate. Asanother example, control module 30 may estimate the heart rate using themedian of the measured R-R intervals or other specific R-R interval inthe group, e.g., the 9^(th) shortest R-R interval. The example heartrate estimation techniques described above provide an estimate of theheart rate that is less susceptible to oversensing while maintainingreasonable sensitivity to short R-R intervals as in the case of VT orVF.

In the example described herein, ICD 20 independently estimates theheart rate on two of the sensing vectors described above with respect toFIG. 1 and compares the estimated heart rates to a tachyarrhythmia heartrate threshold, e.g., a VT/VF threshold. In one example, thetachyarrhythmia heart rate threshold may be set to 180 beats per minute.However, other thresholds may be used. Moreover, in other instances,control module 30 may analyze only a single sensing vector or more thantwo sensing vectors in other instances. Example operation in a “notconcerned” state is described in paragraphs [0064]-[0075] of thespecification as filed and FIG. 7A and FIG. 8 of U.S. Pat. No. 7,761,150to Ghanem et al., entitled “METHOD AND APPARATUS FOR DETECTINGARRHYTHMIAS IN A MEDICAL DEVICE” (referred to herein as Ghanem et al.)The entire content of the referenced portions of Ghanem et al. areincorporated by reference herein in their entirety.

When control module 30 determines that the heart rate on one or both ofthe sensing vectors is above the tachyarrhythmia heart rate threshold,control module 30 transitions to a concerned state 104. In the concernedstate 104, control module 30 discriminates rhythms requiring shocktherapy from those that do not require shock therapy using a combinationof heart rate and ECG signal morphology information. In the concernedstate 104, for example, control module 30 analyzes the morphologymetrics of a plurality of predetermined segments of the sensedelectrical signals and classifies each segment as shockable ornon-shockable. Control module 30 may perform this morphology analysis onthe electrical signals in both sensing vectors in parallel.

In one example, control module 30 analyzes the morphology over aplurality of 3-second segments of the electrical signals and, for eachof the 3-second segments, classifies the EGM in that particular 3-secondsegment as shockable or non-shockable. In other examples, the length ofthe segment analyzed by control module 30 in the concerned state may beshorter or longer than 3 seconds.

The morphology analysis in this concerned state may include a grossmorphology analysis in which metrics are computed for the electricalsignal over the entire segment, without regard for the location of QRScomplexes. The morphology metrics may include, in one example, thesignal energy level, noise to signal ratio, muscle noise pulse count,normalized mean rectified amplitude, the mean frequency, the spectralwidth, and the low slope content. These metrics are exemplary of thetype of metrics that may be used and should not be considered limitingof the techniques described herein. Other gross morphology metrics maybe used in addition to or instead of the metric listed above.

Control module 30 analyzes the gross morphology metrics to classify thesegment as shockable or non-shockable. Control module 30 may analyze oneor more of the gross morphology metrics of the segment to determinewhether the signal in that particular segment is corrupted by noiseand/or artifact. If so, control module 30 may classify the segment asnon-shockable or classify the segment based on the classification of thesame segment in the other sensing vector. If the control moduledetermines that the signal in the segment is not corrupted by noiseand/or artifact, control module 30 analyzes one or more of the grossmorphology metrics to determine whether the signal in the segment is ineither a VT or a VF shock zone and, if so, classifies the segment asshockable. If the segment is determined to not be in the VT or VF shockzone, the segment is classified as non-shockable. Example analysis ofgross morphology during operation in a “concerned” state is described inparagraphs [0076]-[0130] and [0138]-[0141] of the specification as filedand FIGS. 7B-7E, 7H, 7I, FIGS. 9A-9C, FIG. 10, and FIGS. 11A-B of Ghanemet al. The entire content of the referenced portions of Ghanem et al.are incorporated by reference herein in their entirety.

If the gross morphology classification of the segment is shockable,control module 30 may, in some instances, also analyze a morphology ofthe QRS complexes or beats within the segment to classify the segment asshockable or non-shockable. This analysis may be referred to asbeat-based morphology analysis since the control module 30 is onlyanalyzing the morphology of windows around a beat instead of the entiresegment. The window may, for example, have a range between 120-200 ms.In one example, control module 30 may compare the morphology of the beatwithin the window to a predetermined template morphology to determine ifthe beat matches the predetermined template (e.g., has a matching scorethreshold that is greater than or equal to 60%). If more than thethreshold number of beats within the segment, e.g, more than 75% of thebeats within the segment, do not match the template the segment isclassified as shockable. Otherwise the segment is classified asnon-shockable. As such, when gross morphology and beat-based morphologyare both analyzed, the segment must satisfy both analyses to beclassified as shockable. In other embodiments, however, control module30 may make the classification of the segments as shockable ornon-shockable based only on the gross morphology analysis describedabove. One example beat-based morphology analysis of segments of thesensed electrical signal is described in U.S. patent application Ser.No. 14/250,040, entitled “METHOD AND APPARATUS FOR DISCRIMINATINGTACHYCARDIA EVENTS IN A MEDICAL DEVICE USING TWO SENSING VECTORS,”particularly in FIGS. 4, 10, and 11 and the associated description ofthose figures. The entire content of that application is referencedherein in its entirety.

Control module 30 stores the classification of the segments of both thesensing vectors and analyzes the classifications of the plurality ofsegments to determine whether or not to transition to an armed state inwhich capacitor charging begins. If control module 30 determines thatthe rhythm does not require shock therapy (e.g., less than a thresholdnumber of segments are classified as shockable) and the heart rate on atleast one sensing vector is less than or equal to the threshold heartrate, control module 30 transitions to the not concerned state 102. Ifcontrol module 30 determines that rhythm does not require shock therapy,but the heart rate in both sensing vectors is greater than the thresholdheart rate, control module 30 continues analyzing the morphology metricsover subsequent 3-second segments of the electrical signals in theconcerned state 104. If control module 30 determines that the rhythm isshockable during the concerned state 104 (e.g., greater than 2 of 3segments classified as shockable in both sensing channels), controlmodule 30 transitions to an armed state 106.

In the armed state 106, control module 30 initiates charging of thedefibrillation capacitors. Additionally, control module 30 continues toanalyze signal morphology (gross morphology alone or gross andbeat-based morphology) for termination of the shockable rhythm. Controlmodule 30 may, for example, continue to classify segments of the sensedsignal as shockable or non-shockable as described above with respect tothe concerned state 104 and analyze the number of segments classifiedduring either the concerned state 104 or the armed state 106 asshockable. If control module 30 determines that the rhythm requiringshock therapy has terminated, control module 30 returns to the notconcerned state 102. Control module 30 may determine that the rhythm hasterminated, for example, when less than 3 of the last 8 segments areclassified as shockable in both sensed signals and the heart rate in atleast one of the sensed signals is less that the tachyarrhythmia heartrate threshold. If control module 30 determines the rhythm requiringshock therapy is still present once the charging of the capacitors iscompleted, e.g., at least five out of the last eight three-secondsegments are classified as being shockable, control module 30transitions from the armed state 106 to a shock state 108. Exampleoperation in an “armed” state is described in paragraphs [0131]-[0136]of the specification as filed and FIG. 7F of Ghanem et al. The entirecontent of the referenced portions of Ghanem et al. are incorporated byreference herein in their entirety.

In the shock state 108, control module 30 controls therapy module 34 todeliver a shock via a therapy vector that includes defibrillationelectrode 24 and returns to the armed state 106 to evaluate the successof the therapy delivered. For example, control module 30 may determinewhether the tachyarrhythmia has terminated and transition to thenon-concerned state or determine whether the tachyarrhythmia isredetected. The control module 30 may, for instance, redetect thetachyarrythmia when at least 2 of 3 segments classified as shockable inboth sensing channels. Example operation in a “shock” state is describedin paragraph [0137] of the specification as filed and FIG. 7G of Ghanemet al. The entire content of the referenced portions of Ghanem et al.are incorporated by reference herein in their entirety.

One example technique for operating in the non-concerned state, theconcerned state, the armed state and the shock state is described inGhanem et al., which is incorporated by reference herein in itsentirety.

When operating in a detection state in which the morphology metrics ofpredetermined segments of the sensed electrical signal are beinganalyzed, e.g., in the concerned state 104 or the armed state 106 ofFIG. 9, control module 30 may detect a pacing train and, in response todetecting the pacing train, transition to a modified detection state 109in which one or more tachyarrhythmia detection modifications are made.As described above, delivery of pacing by pacing device 16 may interferewith tachyarrhythmia detection by control module 30. Therefore, controlmodule 30 responds to delivery of pacing by modifying thetachyarrhythmia detection analysis to reduce the likelihood ofcorruption. As will be described further with respect to flow diagramsbelow, tachyarrhythmia detection will be modified during the pacingprovided by pacing device 16.

FIG. 11 is a flow diagram illustrating example operation of controlmodule 30 detecting a pacing train and modifying tachyarrhythmiadetection in response to detecting the pacing train. Initially, controlmodule 30 analyzes the pace spike detect signal (or the logicalcombination of the pace spike detect signal and the pace artifact detectsignal) from one or more sensing channels to detect initiation of apacing train (110). In one example, control module 30 detects theinitiation of the pacing train when pace spike detect signal identifiestwo pacing spikes within 1500 milliseconds of one another. In otherwords, the start of a pacing train is detected upon the detection of asingle paced cycle of less than 1500 ms. However, control module 30 mayuse a different threshold than 1500 ms to detect the initiation of thepacing train.

Control module 30 estimates a cycle length of the pacing train (112). Inone example, control module 30 may compute the two most recent cyclelengths of the pacing train using the three most recently detectedpacing spikes and estimate the cycle length of the pacing train as theshortest of the two most recent cycle lengths. This allows for someunderdetection of pacing spikes within the pacing train. For example, if3 out of the last 4 paces are detected, the observed cycle lengths mightbe X and 2X, control module 30 would estimate the cycle length of thepacing train to be X. In other instances, control module 30 may use morethan two most recent cycle lengths (e.g., by using the 3, 4, 5, or moremost recent cycle lengths) or only a single cycle length. Moreover,control module 30 may estimate the cycle length of the pacing trainusing other techniques, such as an average or median of the plurality ofmost recent cycle lengths instead of selecting the shortest of the twomost recent cycle lengths as the estimated cycle length of the pacingtrain.

Control module 30 determines whether the estimated cycle length is lessthan or equal to a first cycle length threshold (114). The first cyclethreshold may be minimum cycle length that may be confidently classifiedas ATP. In one example, the minimum cycle length threshold may be equalto 200 milliseconds. When the estimated cycle length is less than orequal to the first cycle length threshold (“YES” branch of block 114),control module 30 determines that the detected pacing train is likelyEMI and the signal is ignored (116).

When the estimated cycle length is greater than the minimum cycle lengththreshold (“YES” branch of block 114), control module 30 compares theestimated cycle length to a second cycle length threshold (118). Thesecond cycle length threshold may be a maximum cycle length that can beconfidently classified as ATP. In one example, the second cycle lengththreshold may be equal to 330 milliseconds. When the estimated cyclelength is less than or equal to the second cycle length threshold (“NO”branch of block 118), control module 30 determines the pacing train isATP and modifies the detection algorithm to account for the presence ofATP (120). FIG. 12 below describes one example of detectionmodifications made to account for ATP in the sensed electrical signal.In that example, the tachyarrhythmia detection is partially inhibiteduntil ATP has terminated. Other modifications, however, may be made toaccount for the ATP in the sensed signals. In other examples, additionalanalysis other than looking at the estimated cycle length may beperformed to more confidently conclude that the detected pacing trainwith the estimated cycle length is ATP. For example, control module 30may analyze a regularity of the pacing pulse intervals, consistency ofthe pacing artifact amplitude, consistency of the pacing pulse slewrate, and/or consistency of the pacing pulse polarity. Typically, ATPwould be consistent in some, if not all, of these aspects.

Control module 30 continues to analyze the pace spike detect signaland/or the pace artifact detect signal from sensing module 32 todetermine whether the pacing train has terminated (122). For instance,control module 30 may detect that the pacing train has terminated whenone of two conditions are met: (1) a pacing spike has not been detectedfor a threshold period of time or (2) the amount of time since detectingthe initiation of the pacing train exceeds a threshold amount of time.In one example, control module 30 may detect the end of the pacing trainwhen no pace pulse has been detected on the pace spike detect signaland/or the pace artifact detect signal for at least a multiple of theestimated cycle length of the pacing spikes. The multiple may be anynumber greater than 2. In one particular example, the multiple may be2.25 times the estimated cycle length. In other instances, however,control module 30 may utilize a different multiple. Alternatively,control module 30 may detect the end of the pacing train after aparticular amount of time has elapsed from initiation of the pacingtrain. For example, control module 30 may detect the end of the pacingtrain 3 seconds, 4 seconds, 5 seconds, or other predetermined period oftime after initiation of the pacing train. Such a feature sets a maximumduration allowed for detecting ATP.

When control module 30 determines that the pacing train has notterminated (“NO” branch of block 122), control module 30 continues tomodify the detection algorithm to account for the presence of ATP (120).When control module 30 determines that the pacing train has terminated(“YES” branch of block 122), control module 30 reverts to the unmodifiedtachyarrhythmia detection algorithm (124).

Referring back to decision block 118, when the estimated cycle length isgreater than the second cycle length threshold (“YES” branch of block118), control module 30 determines whether the cycle length is greaterthan a third cycle length threshold (126). In one example, the thirdcycle length may be equal to 400 ms. When the estimated cycle length isgreater than 330 ms and less than 400 ms (“NO” branch of block 126), thepacing train cannot be confidently classified as ATP or fast bradycardiapacing based on the estimated cycle length alone. Control module 30 thusdetermines whether there is onset leading up to the pacing or shockablerhythm classification leading up to the pacing (130). If the pacing isATP, it will be preceded by a sudden increase in HR (an “onset”), andlikely will have a shockable rhythm classification for segments prior tothe pacing. In contrast, if the pacing is fast bradycardia pacing, itwill have a slow rise in heart rate over time (i.e., no onset), andlikely will have a non-shockable classification for those segments priorto pacing. In other examples, additional analysis other than looking atonset or rhythm classifications leading up to the pacing may beperformed to more confidently conclude that the detected pacing trainwith the estimated cycle length is ATP. For example, control module 30may analyze a regularity of the pacing pulse intervals, consistency ofthe pacing artifact amplitude, consistency of the pacing pulse slewrate, and/or consistency of the pacing pulse polarity. Typically, ATPwould be consistent in some, if not all, of these aspects.

When control module 30 determines that there is onset leading up to thepacing or shockable rhythm classifications leading up to the pacing(“YES” branch of block 130), control module 30 determines the pacingtrain is ATP and modifies the detection algorithm to account for thepresence of ATP (120). When control module 30 determines that there isno onset leading up to the pacing or non-shockable rhythmclassifications leading up to the pacing (“NO” branch of block 130),control module 30 detects fast bradycardia pacing and modifies thedetection algorithm to account for the fast bradycardia pacing (132). Inone example, a new beat-based morphology consistency discriminator isadded to the tachyarrhythmia detection algorithm. Other modifications,however, may be made to account for the fast bradycardia pacing in thesensed signals. Control module 30 continues to operate in the modifiedbeat-based detection algorithm until the cycle length (e.g., heart rate)of the rhythm falls outside of the VT/VF zone.

Returning to decision block 126, when the estimated cycle length isgreater than the third cycle length threshold (“YES” branch of block126), control module 30 compares the estimated cycle length to a fourthcycle length threshold (128). The fourth cycle length threshold maycorrespond to a maximum fast bradycardia cycle length and may, in oneexample, be equal to 600 ms. When the estimated cycle length is greaterthan the fourth cycle length threshold (“YES” branch of block 128),control module operates in the unmodified detection algorithm. When theestimated cycle length is greater than the fourth cycle length threshold(“NO” branch of block 128), control module 30 detects fast bradycardiapacing and modifies the detection algorithm to account for the fastbradycardia pacing (132).

The thresholds used in the example described in FIG. 11 may be used todetect pacing spike trains of a single chamber pacemaker. The thresholdsmay be different for dual chamber or CRT pacemakers as there may bedifferent timing between paces (e.g., AV delay or VV delay). Otheranalysis techniques may need to be performed for pacing trains providedto more than one chamber of the heart.

FIG. 12 is a flow diagram illustrating example operation of controlmodule 30 implementing a modified tachyarrhythmia detection algorithm toaccount for ATP. Initially, control module 30 detects an ATP train(140). In one example, control module 30 may detect the ATP train whenan estimate a cycle length of the detected pacing train is between200-330 ms or between 330-400 ms with heart rate onset of shockableclassifications immediately prior to the detection of ATP. However, inother examples, control module may detect ATP pacing using differentcycle length ranges.

Control module 30 determines whether the tachyarrhythmia detectionalgorithm has detected a heart rate that exceeds the tachyarrhythmiadetection threshold (142). As described above with respect to FIG. 10,control module 30 operates in non-concerned state 102 in which only theheart rate is analyzed on the selected sensing vectors until the heartrate exceeds the tachyarrhythmia detection threshold, e.g., 180 beatsper minute. When the estimated heart rate on both of the sensing vectorsdoes not exceed the tachyarrhythmia detection threshold (“NO” branch ofblock 142), control module 30 continues to operate in the unmodifiednon-concerned state 102 (144).

When the tachyarrhythmia detection algorithm detects or previouslydetected, e.g., prior to detecting the ATP train, that the heart rateexceeds the tachyarrhythmia detection threshold (“YES” branch of block142), control module 30 is most likely operating in one of the concernedstate 104 or the armed state 106 of FIG. 10. As described above withrespect to FIG. 10, during the concerned state 104 and the armed state106, control module 30 is classifying segments of the sensed electricalsignal as shockable or non-shockable based on the analysis of the grossmorphology of the segments and/or the beat-based morphology within thesegments.

Control module 30 continues sensing on the sensing channels and, ifoperating in the armed state 106, continues charging the defibrillationcapacitors (146). Control module 30 holds all detection state variablesat current states (148). For example, the buffer maintaining the mostrecent, e.g., eight, classifications of the segments as shockable andnon-shockable will be maintained. Control module 30 will ignore anyincomplete segment of the EGM or retrospective segment of the EGM thatincludes the the ATP train (150).

Control module 30 begins a new segment (e.g., 3-second segment) apredetermined period of time after the last detected pace pulse (152).For example, control module 30 may begin a new 3-second segment 330 msafter the last detected pace pulse. In other instances, control module30 may begin the new segment (e.g., 3-second segment) of the signalafter the last detected pace pulse based on the estimated cycle length.Control module 30 determines whether the ATP train has terminated (154).As described above, for example, control module 30 may detect that thepacing train has terminated when one of two conditions are met: (1) apacing pulse has not been detected for a threshold period of time (e.g.,2.25X the estimated cycle length or some predetermined threshold) or (2)the amount of time since detecting the initiation of the pacing trainexceeds a threshold amount of time (e.g., 5 seconds). Note that thecriteria for detecting the end of a pacing train will be met afterinitiation of obtaining the new 3-second morphology segment. In otherwords, the start of a possible 3 second morphology analysis window maybe initiated before the end of a pacing train is detected.

When the end of the pacing train is not detected (“NO” branch of block154), control module 30 ignores the segment of data and a new possiblemorphology segment will again be initiated a predetermined period oftime after the most recently detected pacing pulse (150, 152). Inanother example, control module 30 may not obtain the morphology segment(e.g., 3-second segment) until after detecting the ATP has terminated inblock 154. When control module 30 determines that the ATP train hasterminated (“YES” branch of block 154), control module 30 returns tonormal detection operation and performs the morphology analysis of thenew morphology segment to determine whether the segment is shockable ornon-shockable (156). Control module 30 will therefore update thedetection state as if it were contiguous with the pre-ATP analysis.

FIG. 13 is a flow diagram illustrating example operation of controlmodule 30 modifying a tachyarrhythmia detection algorithm to account forfast bradycardia pacing. Initially, control module 30 detects a fastbradycardia pacing train (160). In one example, control module 30 mayestimate a cycle length of a detected pacing train and detect the fastbradycardia pacing train when the estimated cycle length of the detectedpacing train is greater than 400 ms, as described above with respect toFIG. 11. However, in other examples, control module may detect fastbradycardia pacing using a different cycle length threshold or othertechnique.

Control module 30 determines whether the heart rate as sensed on both ofthe sensing vectors is above a tachyarrhythmia heart rate threshold,e.g., 180 beats per minute (162). When control module 30 determines thatthe heart rate is not above the tachyarrhythmia heart rate threshold(“NO” branch of block 162), control module 30 does not make anytachyarrhythmia detection modifications (164). When control module 30determines that the heart rate is above the threshold heart rate (“YES”branch of block 162), control module 30 implements an additionalbeat-based morphology analysis to monitor the consistency of themorphology. One example scenario that may result in a shockableclassification when no shock is necessary is when the paced evokedresponse results in double counting because of the wide QRS and largeT-waves. The ECG morphology surrounding such a scenario would be anA-B-A-B pattern caused by the consistent oversensing and if the pacingpulses lead to consistent capture.

To identify this scenario, or other scenarios that may causeinappropriate shock classifications, control module 30 compares amorphology of a first sensed event within the current segment with amorphology of a predetermined number of subsequent sensed events withinthe segment and classify each of the comparisons as a match or non-match(166). Each sensed event or beat may be classified as matching when amatching score that is greater than or equal to a threshold, e.g., 60%,otherwise the beat is classified as non-matching. In other instances,control module 30 may compare a morphology of first sensed event afterdetection of ATP with the morphology of the subsequent sensed eventswithin the segments and classify each of the comparisons as a match ornon-match. Whereas the beat-based morphology analysis performed in theconcerned state 104 and the armed state 106 described above in FIG. 10compares the morphology of the beat window to a predetermined templateof an intrinsic heart rate morphology, the additional beat-basedmorphology consistency discriminator compares the morphology of thefirst sensed event of the tachyarrhythmia with morphology of apredetermined number of subsequent sensed events. In one example, thepredetermined number of subsequent sensed events is equal to 11.However, the predetermined number may be greater than or less than 11.

Control module 30 determines whether the number of subsequent sensedevents having morphologies that match the morphology of the first sensedevent of the segment is less than a first threshold (168). In oneexample, the first threshold may be equal to 3 when the predeterminednumber of subsequent sensed events is equal to 11. However, the firstthreshold may be equal to other values, particularly when thepredetermined number of subsequent sensed events is greater than or lessthan 11. When control module 30 determines that the number of thesubsequent sensed events having morphologies that match the morphologyof the first sensed event of the segment is less than the firstthreshold (“YES” branch of block 168), control module 30 characterizesthe segment as shockable if the other gross and beat-based morphologyanalyses indicate shockable (170). This may occur, for example, when thetachyarrhythmia is VF or polymorphic VT.

When control module 30 determines that the number of the subsequentsensed events having morphologies that match the morphology of the firstsensed event of the segment is greater than or equal to the firstthreshold (“NO” branch of block 168), control module 30 determineswhether the number of subsequent sensed events having morphologies thatmatch the morphology of the first sensed event of the segment is greaterthan a second threshold (172). In one example, the second threshold maybe equal to 7 when the predetermined number of subsequent sensed eventsis equal to 11. However, the second threshold may be equal to othervalues, particularly when the predetermined number of subsequent sensedevents is greater than or less than 11.

When control module 30 determines that the number of the subsequentsensed events having morphologies that match the morphology of the firstsensed event of the segment is greater than the second threshold (“YES”branch of block 172), control module 30 characterizes the segment asshockable (170) if the other gross and beat-based morphology analysesindicate shockable. This may occur, for example, when thetachyarrhythmia is a monomorphic VT. When control module 30 determinesthat the number of the subsequent sensed events having morphologies thatmatch the morphology of the first sensed event of the tachyarrhythmia(or segment) is less than or equal to the second threshold (“NO” branchof block 172), control module 30 characterizes the tachyarrhythmia (orsegment) as non-shockable regardless of whether the other gross andbeat-based morphology analyses indicate shockable (174). This may occur,for example, when the detection of the tachyarrhythmia is likely aresult of oversensing.

Various examples have been described. As described above, the conceptswithin this disclosure may be used in implanted systems that do not havean ICD. For example, in an implanted medical system have more than oneleadless pacing device (e.g., a LPD in the atrium and an LPD in theventricle), one or both of the leadless pacing devices may includeperform pace detection, blanking, detection modifications, and the like.This may especially be the case for an LPD configured to detect VT/VFand provide ATP. These and other examples are within the scope of thefollowing claims.

The invention claimed is:
 1. A method comprising: processing electricalsignals sensed via one or more electrodes; detecting, based on theprocessing of the electrical signals, delivery of pacing pulses, whereinthe detecting comprises detecting a first type of pulse having a firstset of characteristics and a second type of pulse having a second set ofcharacteristics different than the first set; modifying the sensedelectrical signal in response to detecting the first type of pulse toremove the first type of pulse from the sensed electrical signal; andmodifying a tachyarrhythmia detection algorithm based on the detectedpacing pulses.
 2. The method of claim 1, wherein detecting the firsttype of pulses comprises detecting the first type of pulses when anamplitude of the sensed electrical signal is greater than or equal to afirst threshold amplitude, and wherein detecting the second type ofpulses comprises detecting the second type of pulses when the amplitudeof the sensed electrical signal is greater than or equal to a secondthreshold amplitude, the second threshold amplitude being less than thefirst threshold amplitude.
 3. The method of claim 1, wherein detectingthe first type of pulses comprises detecting pulses having amplitudesgreater than or equal to 2-10 millivolts and detecting the second typeof pulses comprise detecting pulses having amplitudes greater than orequal to one (1) millivolt.
 4. The method of claim 3, wherein the firsttype of pulses and the second type of pulses have pulse widths of atleast 1 millisecond.
 5. The method of claim 1, wherein modifying thesensed electrical signal to remove the first type of pulse from thesensed electrical signal comprises holding the sensed electrical signalat a current value for a period of time.
 6. The method of claim 5,wherein holding the sensed electrical signal at the current valuecomprises holding the sensed electrical signal at the current value forless than or equal to forty (40) milliseconds.
 7. The method of claim 5,wherein holding the sensed electrical signal at the current valuecomprises holding the sensed electrical signal at the current value forless than or equal to twenty (20) milliseconds.
 8. The method of claim1, wherein modifying the sensed electrical signal to remove the firsttype of pulse from the sensed electrical signal comprises performing aninterpolation between a first value of the electrical signal prior tothe pulse and a second value of the electrical signal subsequent to thepulse.
 9. The method of claim 1, wherein processing electrical signalssensed via one or more electrodes comprises processing electricalsignals sensed via one or more electrodes using a plurality of sensingchannels, wherein detecting delivery of pacing pulses comprisesdetecting the first type of pacing pulses on a first one of theplurality of sensing channels, and wherein modifying the sensedelectrical signal to remove the first type of pulse comprises modifyingthe sensed electrical signal on each of the plurality of sensingchannels to remove the first type of pulse.
 10. The method of claim 1,wherein processing electrical signals sensed via one or more electrodescomprises processing electrical signals sensed via one or moreelectrodes using a plurality of sensing channels, wherein detectingdelivery of pacing pulses comprises detecting the first type of pacingpulses on a first one of the plurality of sensing channels, and whereinmodifying the sensed electrical signal to remove the first type of pulsecomprises modifying the sensed electrical signal to remove the firsttype of pulse from the sensed electrical signal on the first one of theplurality of sensing channels.
 11. The method of claim 1, furthercomprising detecting a heart rhythm that is shockable from the modifiedsensed electrical signal based on the modified tachyarrhythmia detectionalgorithm.
 12. The method of claim 11, further comprising delivering ashock in response to detecting the heart rhythm that is shockable. 13.The method of claim 1, wherein modifying the tachyarrhythmia detectionalgorithm comprises at least modifying a morphology analysis of thesensed electrical signal.
 14. The method of claim 1, further comprising:detecting that the modified sensed electrical signal exceeds athreshold; and detecting a cardiac event indicating an existence of acardiac depolarization in response to the modified sensed electricalsignal exceeding the threshold.
 15. The method of claim 1, furthercomprising: estimating a cycle length of the detected pacing pulses;determining that the cycle length is greater than a minimum cycle lengthand less than a maximum cycle length; and modifying the tachyarrhythmiadetection algorithm in response to the cycle length being greater thanthe minimum cycle length and less than the maximum cycle length.